1. Field of the Invention
The present invention relates to novel processed soybean foods and a method for preparing the same. More particularly, it is concerned with processed soybean foods with reduced phosphorus and potassium, and a method for preparing the same.
Further, it is concerned with starting soybean for said foods and a process for preparing the same.
The foods of the invention are rich in proteins of good quality while containing much decreased levels of phosphorus and potassium as compared with conventional processed soybean products so that it is particularly suitable for use in alimentotherapy for renal diseases.
2. Description of Prior Art
It is traditionally emphasized in alimento-therapy for renal diseases that amounts of food components such as proteins and minerals taken be controlled depending upon the patient's condition. Recently, it has been demonstrated that many patients with renal insufficiencies suffer from complications such as osteodystrophia and hyperlipemia, and development of diets for renal diseases that are capable of preventing or improving such complications is desirable. Soybean is known to possess antihyperlipemic activities because it is rich in vegetable protein and edible fibers of good quality and further it contains unsaturated fatty acids. Soybean is therefore suitable for alimentotherapy for renal diseases. On the other hand, soybean contains potassium as high as ca. 2,000 mg and phosphorus as high as ca. 600 mg per 100 g of the edible portion. In patients with renal diseases, serum potassium level increases due to their insufficient potassium excretion, which is occasionally accompanied by dyspnea and paresthesia in extremities, and in the worst cases, heart arrest. Serum phosphorus level also increases in such patients under the similar condition of potassium, and causes disorders of calcium metabolism such as osteodystrophia. In prior art processed soybean foods, which are prepared by using ordinary soybean, satisfactory reduction in phosphorus and potassium levels has not resulted.
Therefore, processed soybean foods could become a more desirable diet for renal diseases by using the soybean with a selectively decreased phosphorus and potassium content.
Phosphorus in soybean is contained in the form of phytin in a portion of ca. 75% and of phospholipid in a portion of ca. 13%. When soybean is defatted, a part of the phospholipid is separated together with the oil and fat components, but the phytin-form phosphorus and potassium remains together with water-soluble components such as proteins in the defatted soybean. Phytin is considered to be an antinutritional substance which inhibits absorption of calcium and others derived from food, and a number of methods have heretofore been developed for decreasing the phytin content of soybean. However, the prior art methods are not aimed at decreasing the content of the phosphorus itself nor made in consideration of the potassium content. For example, in Japanese Patent Public Laid Open No. 166049/1984 a method is described in which phytin is removed by hydrolyzing the phytin (phytic acid) to form inositol and phosphoric acid. The content of phosphorus itself is not decreased by this method. There is described in Japanese Patent Public Laid Open No. 7300/1974 a method of separating phytin by such means as using an ultrafiltration membrane from soybean which has been pulverized in an aqueous solution. As phytin in solution is apt to be adsorbed on protein, efficient removal of the phytin in solution by this method is difficult, and operations of the method are also complicated.
As a result of extensive studies to develop a method of efficiently removing the phosphorus and potassium components from soybean by simple procedures, we have found that, by merely soaking soybean in an aqueous solution at pH 3.0-6.5, the phytin in soybean is hydrolyzed to inositol and phosphoric acid, the phosphoric acid is eluted into the aqueous solution and the potassium component in soybean is simultaneously eluted thereby producing processed soybean with the two components decreased.